Firefox 91 release overview: new ESR base, Simplified Printing, new HTTPS-First Policy

Firefox 91 is the latest stable version of the web browser. Released on August 10, 2021, it is replacing Firefox 90 and previous versions of the stable branch of the browser.

All other Firefox release channels have been updated as well. Firefox Beta and Developer are moved to version 92, Firefox Nightly to version 93, and Firefox for Android will be moved to version 91 as well (a bit later, usually.

Firefox ESR is moving to a new base. Firefox 91 ESR is the new base which will replace Firefox 78 ESR in October (both run parallel for two releases).

Check out the Firefox 90 release overview to read up on the changes in Firefox 90.

Executive Summary

• Firefox 91 is the new base for Extended Support Releases. It is supported for about a year with the released feature set.
• Support for Windows single sign-on added.
• HTTPS-First Policy in Private Browsing mode.
• Firefox ESR 78.x is the last Firefox version that supports Flash officially. Support for it ends on October 10, 2021.
• Firefox ESR 78.x is the last that supports Mac OS 10.12 or older.

Firefox 91.0 download and update

Firefox 91.0 is rolled out to all desktop systems starting August 10, 2021. Most Firefox installations are configured to download and install browser updates automatically.

You may select Menu > Help About Firefox to run a manual check.

Manual downloads are also available. The following pages list direct downloads for supported Firefox channels.

• Firefox Stable download
• Firefox Beta download
• Nightly download
• Firefox ESR download
• Firefox for Android on Google Play

New features and improvements

HTTPS-First Policy

Firefox will try to connect using HTTPS first when in private browsing mode. The browser falls back to HTTP only if a HTTPS connection is not available.

Firefox supports a HTTPS-Only Mode which users may configure to prioritize HTTPS always. The browser displays a warning page if HTTPS is not supported by a server. Options to use HTTP are then provided.

Firefox users who enable HTTPS-Only Mode in private browsing mode, or in all modes, get the same functionality.

Simplify Printing restored

Mozilla introduced a new printing interface in Firefox 81. Simplified Printing, an option to turn the content into a bare-bones version optimized for printing, was removed in that update.

Simplified printing displays a version of the page based on the browser's Reader View feature. The focus is on the article and related content such as images; menus, advertisement, other blocks on the site are not displayed in the mode.

Firefox 91 restores the option, and you may once again select it in the print dialog.

You can check out our full overview of simplified printing in Firefox 91 here.

Other changes

• Firefox supports signing-in to Microsoft work and school accounts using Windows single sign-on (on Windows, obviously).
• Total Cookie Protection improvements that update the cookie clearing logic and reveal to users which websites are storing information locally.
• New locale Scots (sco) added.
• Firefox enables High Contrast Mode on Mac OS automatically if "Increase Contrast" is checked.
• 10%-20% increase in most user interactions thanks to "catch-up paints".
• New file downloading behavior. All downloads are saved to the default directory or selected directory, even if "open" is selected.
• Switch to Tab results in Firefox's address bar in Private Browsing windows (previously only in regular browsing windows).

Developer Changes

• The Gamepad API requires a secure context starting in Firefox 91.
• Visual Viewport API is supported by desktop versions of Firefox.
• Intl.DateTimeFormat.prototype.formatRange() and Intl.DateTimeFormat.prototype.formatRangeToParts() are now supported in release builds
• The Error() constructor can now take the error cause as value in the option parameter.
• Window.clientInformation has been added as an alias for Window.navigator.

Enterprise changes

• Support for Enterprise policies for Linux Snap users.
• WindowsSSO policy added to enable or disable the Single Sign-On feature on Windows.
• SearchEngines policy update supports setting the query charset. It defaults to UTF-8 instead of windows-1252 now.

Known Issues

None listed.

Security updates / fixes

Security updates are revealed after the official release of the web browser. You find the information published here after release.

Outlook

Firefox 92 will be released on September 7, 2021. Two Firefox 78.x ESR versions will be released in the coming months: Firefox 78.14 ESR in September, and the last 78.x release, Firefox 78.15 ESR in October.

Firefox 91 release overview: new ESR base, Simplified Printing, new HTTPS-First Policy